1. Field of the Invention
This invention relates to electrical heating systems.
2. Introduction to the Invention
Elongate electrical heaters are often used to heat systems of pipes and/or storage vessels which contain liquids and which are thermally insulated. Most such systems fall into two different categories. In the first category, the liquid is water, and the objective is to ensure that the water does not freeze. In typical freeze-protection systems in this category, one or more heaters are controlled by a single thermostat which senses the ambient air temperature and switches the heater(s) on if the ambient air temperature falls below a lower limit temperature, e.g. 40.degree. F., and switches the heater(s) off if the ambient temperature increases to a higher limit temperature. e.g. 43.degree. F. One disadvantage of such systems is that the heater is switched on continuously even though this is not required (except when the ambient temperature is at the lowest value at which the heater can prevent the water from freezing). Another disadvantage is the loss of control if the thermostat fails, which may not be immediately apparent. Another disadvantage is the cost of the thermostat, especially when the current drawn by the heater(s) is large, either in the steady state or when the heater is first switched on.
In the second category of systems, the liquid is not water (e.g. it is an organic liquid or a solution, slurry, suspension, or dispersion in water or an organic liquid) and the objective is to maintain the liquid at a relatively high temperature, e.g. 45.degree. F. or more, as it is moved through the system from one storage vessel or treatment station to another. Such systems are used, for example, in hydrocarbon refineries and food processing plants. In typical temperature-maintenance systems of this type, there are a number of different sectors, and each sector is heated by a different heater. Each heater is controlled by a line-sensing thermostat which senses the temperature at a selected point on one of the pipes heated by that heater. Once the sector has been heated to the maintenance temperature, the heater is switched on and off during successive and alternating heat-up and cool-down periods, and the duration of those periods is controlled by the line-sensing thermostat. The line-sensing thermostat turns the heater on when the sensed temperature falls to a lower limit, and turns the heater off when the sensed temperature reaches an upper limit. Temperature-maintenance systems require very careful design, in particular when the flow pattern may be deliberately changed, thus producing different thermal demands. The system must be divided into sectors which can appropriately be heated by a single heater, and within each sector, the line-sensing thermostat must be placed at the most vulnerable part (i.e. the part most likely to drop below the maintenance temperature in any possible flow pattern within the sector). The most vulnerable part is usually the smallest pipe in the system and/or a pipe in which there may be no liquid flow. In many temperature-maintenance systems, there are more than 5 sectors, each with its own heater, and systems containing more than 20 sectors are by no means unusual. In at least some of the sectors, the heater may be quite short, e.g. less than 20 feet long, and the average circuit length (total length of all the heaters divided by the number of sectors) may be 25-40 feet. These systems suffer from a number of disadvantages, for example the cost of the thermostats, of placing the thermostats at the most vulnerable parts of the respective sectors, and of running appropriate wiring to the thermostats; the loss of control if a thermostat fails (which may not be immediately apparent); the difficulty of replacing a failed thermostat, especially if the most vulnerable part of the sector is not readily accessible; and the need to reposition at least some of the thermostats if the system is changed or a new flow pattern is adopted.